1. Field of the Invention
The present invention relates to a process for producing a binder resin for a toner which comprises a plurality of resins.
2. Description of Related Art
A variety of polymers have been developed and made use of in various fields as, for example, base materials, coating materials, binders, etc. The polymers can be used individually for the respective purposes but, in many cases, where desired characteristics cannot be sufficiently obtained from a single polymer, they have been used as a mixture of two or more thereof to compensate for the insufficiency.
For example, many binders for a toner for developing an electrostatic latent image comprise a mixture of a high-molecular weight polymer and a low-molecular weight polymer because they are required to have various characteristics. Such binders involve the following problems.
A toner for developing an electrostatic latent image is generally made up of a resinous component, a colorant (a pigment or a dye), and additives, such as a parting agent and a charge control agent. The resinous component includes natural or synthetic resins used either individually or as an appropriate mixture thereof. Many improvements have been proposed on the resinous component of a toner suited to a dry development system, which has recently undergone rapid technological development. In particular, a fusing roller system has been adopted in an electrophtographic copying machine aiming at high-speed copying and low-temperature energy-saving copying. According to this fixing system, a toner image formed on an electrostatic recording medium called a photoreceptor is transferred to a transfer sheet, such as paper, and the sheet is passed through fusing rollers for hot pressing thereby to fuse and fix the toner image onto the sheet.
Various methods for embodying a fixing system are known. A contact heat fixing system typically including a system using a fusing roller unit is superior in thermal efficiency to a non-contact heat fixing system using, e.g., a hot plate fixing unit, and is preferred particularly for feasibility of fixing at a high speed and a low temperature.
However, if the contact heat fixing system is applied to conventional toner, the toner coming into contact in a molten state with the fusing roller is transferred onto the fusing roller and stains the next transfer sheet (called an offset phenomenon).
On the other hand, copying machines have ever been increasing the speed, and it naturally follows that the contact time with a fusing roller becomes shorter. Therefore, a toner that fuses in a short heating time has been demanded. Further, from the standpoint of energy saving and safety, a toner that fuses at as low a temperature as possible and shows satisfactory fluidity on fusing has been sought.
In order to securely accomplish fixing at a fixing temperature, it is accepted preferable that the toner for this fixing system contains a binder resin comprising a low-molecular weight polymer so as to decrease the viscosity and also a high-molecular weight polymer so as to increase the modulus of elasticity and to prevent the offset phenomenon (sticking of part of the toner to a contact fusing roller).
Styrene resins are often used as such a binder resin for a toner which comprises a low-molecular weight polymer and a high-molecular weight polymer, and various methods of polymerization have been studied.
For example, Japanese Patent Laid-Open No. 48657/90 discloses a method in which a high-molecular weight polymer is produced by suspension polymerization using a polyfunctional initiator, and a low-molecular weight polymer is then produced in the presence of the high-molecular weight polymer. The resulting polymer is dried to provide a solvent-free polymer mixture comprising a high-molecular weight polymer and a low-molecular weight polymer, which is expected to be applicable as a binder resin for a toner.
In general, it is relatively easy to obtain a high-molecular weight polymer by suspension polymerization using a crosslinking agent, such as divinylbenzene, diethylene glycol dimethacrylate, and trimethylolpropane dimethacrylate. However, in order to obtain a low-molecular weight polymer by suspension polymerization, it is necessary to use a large quantity of a chain transfer agent, such as mercaptans or halogen compounds, or a large quantity of a polymerization initiator. In using a chain transfer agent, the polymer must be subjected to post-treatment to remove an undesired odor or a residual halogen compound, which increases the cost. Further, the method involves difficulty in removing unreacted polymerizable monomers.
Japanese Patent Laid-Open No. 48657/90 discloses a technique comprising dissolving a low-molecular weight polymer obtained by solution polymerization in a polymerizable monomer which is to provide a high-molecular weight polymer and causing the system to polymerize by addition of a polyfunctional initiator (having at least trifunctionality) to prepare a binder resin for a toner. However, a solution polymerization system for producing a high-molecular weight resin produces the Weissenberg effect (a phenomenon that a resin rises, clinging to a stirring rod), which makes the production difficult.
U.S. Pat. No. 5,084,368 teaches combining solution polymerization with bulk polymerization for production of a high-molecular weight polymer.
Any of these processes based on solution polymerization requires a step of solvent removal for obtaining a solventless resin mixture, which not only needs labor but incurs cost.
Additionally, since solution polymerization has a difficulty in producing polymers having a high weight average molecular weight of more than 200,000, the resulting polymers have not sufficient molecular weight enough to overcome the problem of offset.